Answer:
In the context of classical conditioning, closing the door has become a(n) conditioned stimulus.
Explanation:
A conditioned stimulus, in classical conditioning, can be defined as a previously neutral stimulus that has become associated with an unconditioned stimulus, beginning to trigger a response.
It is very likely that closing a door didn't mean much to students in general (neutral). However, it was paired with the announcement of a quiz. Announcing a quiz (unconditioned) triggers a response - students feel surprised, anxious, worried, etc. After pairing up both stimuli a few times, students learn to associate them. Now, closing the door is a conditioned stimulus. Even if the professor does not announce a quiz, students will feel anxious and worried just by watching him close the door.
